The present invention relates to an improved disk for magnetic recording, and more particularly relates to an improvement in composition of a disk used for magnetic rotary encoders.
Among various rotary encoder used as indispensable sensors for servo-control systems, a magnetic rotary encoders is well known in which signals magnetically recorded on the peripheral fringe of a disk made of magnetic material are read out by a magnetic sensor for detection of amount of rotation of a motor or the like.
Fe-Cr-Co alloys have conventionally been used for such disks because of their hardness and high magnetic property and excellent workability. Due to increasing demand in market for higher resolving power, larger diameter constructions have been recently employed. This has made it necessary to use thinner disks in order to minimize the increase in inertia during rotation of the disk which would otherwise result from the increased diameter of the disks.
The trend to larger and thinner disks tend to result in warping of the disk both because the material for the disk has an inherent tendency to warp and because of grinding operations which take place during production of the disk. The presence of such warps causes variations in the gap between the disk and an associated magnetic sensor as the disk rotates for read-out, thereby significantly enlarging read-out errors.
For example, in the case of a disk for a rotary encoder whose periphery or peripheral fringe is magnetized for formation of a magnetic calibration (magnetic signals) at a pitch of 30 to 100 .mu.m, the degree of warping has to be minimized to 5 .mu.m or less when a magnetic sensor is 2 to 10 .mu.m from the face of magnetization on the disk. When the degree of warping exceeds this limit, the resultant variation in the gap unavoidably produces read-out errors in the form of changes in the output voltage.